(i) For a rhombus and a parallelogram, adjacent angles are supplementary and opposite angles are equal.
For both these shapes, the statement is true as the sum of adjacent angles for these shapes is 180° and opposite angles are equal.

(ii) If all the angles of a parallelogram become equal, then the shape thus formed is a square.
It is false because it can be a rectangle or a square and we do not have any information about the length of the sides whether they are equal or not; because for a square, all the sides must be equal.
(iii) A trapezoid is isosceles if and only if the base angles are congruent.
Let ABCD be a trapezoid and AB || CD, AD = BC and DA || CE.
Hence, ADCE thus formed is a parallelogram.
Now, DA = CE and DC = AE (Properties of parallelogram)
If BC ≅ CE, then angles opposite to them are congruent.
∠CEB ≅ ∠CBE
So, by property of parallelogram and linear pair angles,
∠DAB ≅ ∠ABC
Interior angles on the same side of the transversal are supplementary.
Hence, ∠A + ∠D = 180° and ∠B + ∠C = 180°
=> ∠A + ∠D = ∠C + ∠B
=> ∠D = ∠C